A semiconductor integrated circuit available in the prior art is generally provided with a protection circuit arranged between a terminal and an internal circuit proper thereof for preventing breakdown of the internal circuit proper caused by electrostatic electricity generated therein, from occurring. An example of such protection circuits comprises diodes and resistors employed in combination, including a first diode which is a junction produced in an N-type semiconductor layer (Hereinafter such a junction is referred to as a P-type diffusion layer.) and a junction produced in a P-type semiconductor layer (Hereinafter such a junction is referred to as an N-type diffusion layer.).
For example, the first diode is arranged to connect its P-type diffusion layer with both the internal circuit and the terminal of the semiconductor integrated circuit, so as to allow a positive surge voltage applied to the terminal to bypass the P-type diffusion layer and to flow into the semiconductor substrate, thereby protecting the internal circuit proper from the positive surge voltage.
As for the second diode, its N-type diffusion layer is connected the internal circuit and the terminal, while its P-type diffusion layer is connected a low potential side. Consequently, a negative surge voltage applied to the terminal is allowed to bypass the N-type diffusion layer and to flow into the P-type diffusion layer, thereby protecting the internal circuit proper from the negative surge voltage.
In addition, resistors are connected respectively to the first and second diodes to protect the diodes from the surge voltages which are respectively applied thereto from the opposite directions.
In recent years, in conjunction with a trend towards higher integration of semiconductor devices, semiconductor devices are inclined to become smaller in size and the wirings are becoming more minute. In such advanced semiconductor devices which are enhanced in integration, the breakdown voltage characteristics with respect to surges such as the ones caused by discharge of electrostatic electricity, are tending to decline. However, even if advance is made to create a new-generation of semiconductor devices, surges applied thereto from the terminal do not decrease, and protection circuits are required to be provided with the characteristics for absorbing a certain range of excess voltage. In the protection circuits having the above-described circuit configuration, in order to maintain satisfactory characteristics, the diodes and the resistors require some dimension. For this reason, the horizontal area of the protection circuits inevitably increases to some extent. This is one parameter which hampers integration of semiconductor devices from being enhanced.